Asbestos process

ABSTRACT

A process of recovering asbestos fibre from asbestos-bearing material comprising a first stage of preparing a fibre concentrate by the steps of comminution, classification and spiral concentration and a second stage of liberating and recovering fibre, wherein in the second stage there is a separation by means of a hydrocyclone into a well-opened fraction and a poorly opened fraction, and wherein the asbestos-bearing material is substantially moist throughout the process.

TECHNICAL FIELD

This invention relates to processes for recovering asbestos fibre fromasbestos-bearing materials. In particular it relates to wet processesfor treating asbestos-bearing materials.

BACKGROUND ART

The commercial recovery of chrysotile from its ores is typically carriedout in a plant in which the asbestos fibre is separated from its hostrock by means of a pneumatic system. A complex plant is required, inwhich a complex sequence of stages of comminution and fibre removal isemployed. Despite this, the extraction of the fibre content of some oresis inefficient and a considerable amount of fibre may be left in thetailings from the process. In addition since the longer fibre gradescommand a premium price, there is frequently considerable loss ofproduct value by the breaking of long fibres into short fibres duringthe comminution stages.

It is usually necessary to dry the ore feed to the conventionalpneumatic process. The drying operation and subsequent process steps arepotential sources of release to the environment of chrysotile-containingdust, with its associated health risks. The statutory authoritiescharged with safeguarding community health in various countries haveestablished maximum allowable concentrations of respirable asbestosfibres in the atmospheres in which people are required to work. Theseconcentrations limits have been made progressively more stringent inrecent years and consequently it is becoming more difficult for dryextraction plants to comply with them.

A means of reducing the possibility of emission of asbestos dusts to theatmosphere is to carry out the various operations in an aquoues mediuminstead of in air. However, it has not proved feasible, hitherto, todevise a wet process having the efficiency, economy and generalapplicability required for commercial development.

A wet process for separating asbestos from its ores, for example, hasbeen disclosed in U.S. Pat. No. 4,226,672. This process involves the useof an aqueous medium containing a chemical reagent as an aid in thefiberizing step and to improve the quality of the product fibre. Inaddition to providing a less dusty and hence less hazardous workingenvironment, and an improved yield compared to conventional dry process,the process of U.S. Pat. No. 4,226,672 can provide a fibre of particularreinforcing value in the manufacture of asbestos cement products, suchas asbestos cement sheet. This process, however, has higher costsassociated with the use of the chemical agent and in the removal ofchemical agent from process liquors for recycling in the process.

Thus it is an object of our invention to provide a wet process forrecovering asbestos fibres from a wide range of asbestos-bearingmaterials, for example, crude ore, mill feed, selected streams fromconventional processes, tailing, mine wastes and low grade short fibre.

DISCLOSURE OF INVENTION

We have now found a wet process for recovering asbestos fibres fromasbestos-bearing materials that allows the said materials to bemaintained in a substantially moist condition throughout the processing,and provides yields of fibre at least equal to, and typicallysubstantially greater than, those obtain by the application ofconventional dry process to the same asbestos-bearing materials. It is aparticular feature of our process that it may be applied to intractableasbestos-bearing materials that cannot be satisfactorily treated by theconventional dry process.

Accordingly we provide a wet process for the recovery of asbestos fibrefrom asbestos-bearing material which process comprises a first stage ofpreparing a fibre concentrate by the steps of comminution,classification and concentration and a second stage of liberating andrecovering fibre, characterized in that in the first stage,concentration is by means of spiral classifiers, and that in the secondstage there is a separation by means of a hydrocyclone of less than 150mm cross section into a well-opened fraction and a poorly openedfraction, and wherein the separated fibre is recovered on a screen.

Preferably the screen is of the sieve-bend type. In a more preferredembodiment the underflow is subjected to further comminution, typicallyin a rodmill, and again treated with a hydrocyclone.

The use of spiral classifiers and hydrocyclones has found littleapplication in the commercial processing of asbestos-bearing materialsand therefore it is surprising that this particular combination providesnot only a viable wet process but in most cases a substantial increasein yield. Reference to yield here is to the composite value of theproduct derived from the actual yield weight and quality grade of fibre.

BEST MODE OF CARRYING OUT THE INVENTION

An important commercial objective for processing of asbestos ores is themaximization of the value of the product fibre which can be economicallyextracted. The nature of asbestos ores varies widely, particularly withrespect to the preparation and characteristics of the chrysotilecomponent. The method of our invention offers the technical flexibilityfor dealing economically with such variations. An important advantage ofthe present invention is its capability for recovering valuable fibrefrom the tailings discarded from conventional dry milling operations.The typical general flow diagram for the process of our invention isgiven in FIG. 1 and our process will now be discussed with reference tothis diagram but it is not intended that our invention be limited tothis particular flow diagram.

Before the chrysotile fibre can be concentrated from an asbestos ore itmust be adequately detached from the host rock. This can be achieved bycontrolled comminution. In order to minimize the degradation ofpotentially good quality fibre by excessive exposure to crushing andgrinding devices, it is an important aspect of our process that theliberated fibre is separated from the bulk of the rock stream at theearliest opportunity in the treatment scheme.

The comminution is achieved in a sequence of a main stage (1) applied tothe bulk of the crude ore and a subsequent stage (3).

Each comminution stage employs a crushing or grinding device appropriateto the particle size range required and which can accept a solid feedwhich is at least wet but may be in the form of a slurry. In addition tothe detachment of fibre from rock, some opening of fibre bundles iseffected which assists the subsequent hydraulic separation operations.The selection of the types of crushing and grinding equipment which canbe used for achieving size reduction below any particular particle sizelimits involves both economic and technical constraints.

Crushers of impact or compression types can be used to reduce theparticle size of the feed material to below about 10-15 mm. The primarycrushing of the wet solid can be effected using a jaw crusher, forexample. The secondary crushing, optionally with the addition of furtherwater with the solids, can be carried out by means of a cone crusher,for example.

At this first stage (1) of comminution our process provides for removalof the fibre (2) which has been substantially liberated from rock,producing a first crude fibre concentrate. We use for the separation ofthis concentrate a mechanical classifier (2) such as a screw classifier.This crude fibre concentrate (A) is further enriched by presenting it toa concentrating device (7), preferably a spiral concentrator.

Alternatively, where there is a relatively high concentration of goodquality fibre in the feed material, the separation of the first crudefibre concentrate can with advantage be carried out using a large,preferably greater than 150 mm, low-pressure hydrocyclone instead of theclassifier at (2). This produces an overflow stream containing fibrecomparatively free of rock particles larger than the cut point of thehydrocyclone. For a d₉₅ cut point of 25 microns, for example, the fibrewill be comparatively free of rock particles with a longest dimensiongreater than about 30 microns. By a particular d₉₅ cut point we mean thesize at which 95% of particles of that size in the feed report to theunderflow can be given a scavenging treatment to recover further fibreusing a spiral concentrator, the fibre accumlating in the lightfraction. The heavy or gangue solids presented to the spiralconcentrators are preferably smaller than 1.5 mm in their longestdimension for efficient operation.

The stream remaining after removal of the crude fibre concentrate (A) issubjected to further stages of comminution (3), using grinding mills inwhich the grinding medium may be rods, balls, slugs, or pebbles, toreduce the particle size of the heavy fraction (rock stream) from themechanical classifier and prepare it for presentation to theconcentrating device (4), preferably a spiral concentrator. The heavy(rock) fraction from this spiral concentrator can be given a furtherscavenging treatment. The rock stream is presented to a screen (5),preferably a sieve bend, having an aperture of about 300 microns. Theoversize (D) from this screen is passed again through a ball mill (3)and then passed over another spiral concentrator to produce a furtherfibre concentrate which is combined with the previously fibreconcentrate (B) and submitted to a cleaning operation (7).

The undersize from the screen or sieve bend (5) comprises mainly rockparticles (E) typically of a size which it is considered noteconomically useful to grind further. The small amounts of short fibreand composite particles present are removed from this fine rock streambefore it is discarded from the system, by passing over another spiralconcentrator. The rock tailings slurry is sent to a thickener (6) fromwhich the clarified water may be re-used in the circuit.

By selection of a suitable section profile for the spiral concentratorsthe incoming solids can be separated into three fractions. The more openfibre is included in a light fraction, the less open or spicular fibreis included in a middlings fraction, which may be worth some furthertreatment to release more open fibre, and the substantially non-fibroussolids reports to a heavy fraction. Spiral concentrators have been foundto be particularly suitable for rejecting essentially barren rockparticles or grit (larger than about 200 mesh) from the treatmentcircuit. It will be understood that where one spiral concentrator isreferred to, it may be necessary in practice to effect multiple passesof selected process streams through several spiral concentrators inseries, in order to ensure that the required efficiency of fibrerecovery is attained. Different section profiles can be selected forbest effect with different rock size ranges.

The crude fibre concentrates (A), (B), from which the grit has beenremoved in spiral concentrators, are further upgraded by passing throughlarge (preferably greater than 150 mm), low-pressure hydrocyclones (7)which largely eliminate into the underflow (F) the rock particles largerthan the cut point of the hydrocyclones (typically about 30 micron).

The cleaned fibre concentrate from the large hydrocyclone overflow isfractionated into a well-opened fraction (B) and a poorly-opened orspicular fraction (H) by passing through small (typically 50 mm), highpressure hydrocyclones (8). The resulting well-opened fibre fraction ispresented to a fine screen (9), typically with a 100 micron opening, theundersize from which is valueless fine gangue, the oversize being asubstantially dewatered fibre product of commercial quality (product J).The fine screen is preferably a sieve bend; a unit which is pressure fedand rapped periodically is especially effective.

The underflow (H) from the small hydrocyclone comprises substantiallyunopened fibre particles, pencils, spicules and the like. For some oresthis fraction can represent a major proportion of the total fibrecontent, consequently, it is important to optimize its quality. This isachieved by fiberizing or opening the fibre bundles in a suitablefiberizing device (10), such as a rod or ball mill or a rotating disctype of colloid mill. Again the fiberized product is passed through asmall high pressure hydrocyclone, to eliminate in the underflow anyunopened fibre particles for reprocessing. The overflow is again cleanedby presenting it to a fine screen (11), which effects the removal offine gangue as well as substantial dewatering of the fibre Product K.

Optionally, the unopened fibre fraction from the small hydrocyclone canbe fiberized with the assistance of added chemical reagents. By theapplication of the chemical fiberizing treatment described in U.S. Pat.No. 4,226,672, for example this fiber can be upgraded to form a fibreproduct, with a high reinforcing value in asbestos cement. The selectivechemical treatment of the unopened fibre fraction, and the optionalsubsequent blending of this product with the previously separated openedfibre fraction, allows the impact of the cost of the chemical treatmentto be minimized by the capability of this method to eliminate fine rockparticles from the fibre, since these unnecessarily consume chemical inthe process of U.S. Pat. No. 4,226,672.

Thus, in a further embodiment of the invention, the poorly-opened fibrefraction is further opened in a suitable fiberizing device with the aidof a chemical fiberizing reagent.

The partially dewatered product (J, K) is preferably further dewateredfor use and economy of storage and transport. This can be effected bysubjecting the partially dewatered product (J, K) to pressure. Theequipment used for applying the pressure can be chosen in accordancewith the final physical form required for the product. For example,pressure may be applied in a briquetting machine. Alternatively, thepreliminary dewatering can be effected in equipment such as is used forforming asbestos paper or board and the sheets of wet fibre from theseunits further dewatered as required by subjecting to suitable pressures,for example by passing between compaction rolls.

Whereas conventional pressure filters or centrifuges typically do notreduce the moisture content of the product fibre much below 40% byweight, it is possible to achieve moisture levels down to about 17% byweight of application of pressures in the range illustrated by Examples6 to 10. It is essential that all of the fibre should remain wet at alltimes in order to avoid the possible generation of airborne dust. Thiswould not be the case with other commercially practicable and economicmethods for lowering the water content, such as those involving theapplication of heat or passage of gases, in which it would be difficultto avoid making some part of the fibre too dry.

Thus in a further embodiment of the process of our invention we providea process of dewatering wet asbestos fibres by filtration under highpressures. Typical suitable apparatus for this high pressure filtrationincludes, for example screw presses and V-presses. A particularadvantage of these presses is that the dewatered asbestos is recoveredin the form of pellets which may be conveniently packed and shipped.

We have further found that the wet asbestos fibres can be effectivelydewatered by an electrokinetic technique.

INDUSTRIAL APPLICABILITY

It is a particular feature of the process of our invention that itprovides a means of recovering asbestos fibre from asbestos-containingmaterial that is relatively free of the dust hazards associated with theconventional. Furthermore the yields obtained by our process from agiven crude ore are substantially greater than those obtained from theconventional dry process from comparable samples of the same ore. In thecase of intractible ores that are difficult to treat by the dry processthe increase in yield value has been as great as 100%, since the wetprocess minimizes both the loss of fibre through crushing to finefragments and dust, and more generally, by reducing the loss of fibrevalue through breakage of long high value fibre.

The asbestos fibres produced by our process may be used in all theconventional asbestos fibre applications. In particular the dewateredproduct containing 17-18% by weight of water, and whether in the form ofpellets, briquettes or "paper rolls" is in a convenient form for makingcementitious compositions, such as for example, asbestos cementarticles. The product readily disintegrates when immersed in water andis then easily redispersed as required.

The invention is now illustrated by, but not limited to, the followingexamples. All parts and percentages are on a weight basis unlessotherwise stated.

EXAMPLE 1

Stage 1.

Approximately 300 kg of crushed ore from Woodsreef mine (passing 16mesh) was treated in a semicontinuous demonstration unit according tothe general flow diagram of FIG. 1.

The ore was fed at 2-3 kg/min into a sump where it was mixed with waterand the slurry pumped to the rough fibre concentration Stage 2.

The operations carried out and the key items of equipment used for eachstage of the process are described below.

Stage 2. First rough concentration of fibre

The primary separation of fibre and rock particles was effected in a 150mm hydrocyclone. A spiral concentrator was used to scavenge furthercrude fibre from the 150 mm hydrocyclone underflow (rock stream).

Stage 3. Comminution

Further liberation of fibre from rock and opening of fibre bundles wascarried out by subjecting the spiral tails from Stage 2 to grinding in arod mill.

Stage 4. Second rough concentration of fibre

After grinding, a further crude fibre concentrate was separated from thesubstantially rock stream in a spiral concentrator. The configuration ofthis spiral concentrator allowed the separation of a middlings fractionas well as a tailing fraction. The middling fraction was recycled.

Stage 5. Sizing

The rock tailing stream from the spiral concentrator of Stage 4 waspresented to a weir-fed 45° sieve bend of 100 micron aperture, theoversize from which was recycled to receive further comminution in Stage3.

Stage 6. Waste dewatering

The undersize stream from Stage 5 was passed to a lamellar thickener inwhich the fine gangue was separated from the bulk of the water forrejection as thickened waste. The water was recycled to the initialoperations in the process.

Stage 7. Cleaning fibre concentrates

The rough fibre concentrates from the spiral concentrators in Stage 2and 4 were cleaned by passing through a 150 mm hydrocyclone. The gritseparated in the underflow was re-treated to extract residual fibre, asin Stages 3 and 4, by passing through a ball mill and then over spiralconcentrators.

Stage 8. Fibre fractionation

The fibre rich stream produced as the overflow from the 150 mmhydrocyclone (Stage 7) was fractionated, to separate the well-openedfibre from the poorly-opened fibre, by passing through a small (50 mm)hydrocyclone. The poorly opened fraction was upgraded by removing gritin spiral concentrators, the concentrate from which represented anintermediate fibre product requiring further opening to attain itsoptimum value for reinforcing cement. The tailings fraction from thesespiral concentrators was retreated as in Stages 3, 4 and 5 to extractany remaining useful fibre.

In order to ensure the high cement reinforcing value sought in thisexperiment, the poorly-opened fibre fraction was further enriched byretreatment through spiral concentrators to eliminate residual grit, andsieve bends to remove fine gangue.

Stage 9. Fibre Cleaning

The opened fibre in the small cyclone overflows from Stage 8 was furtherupgraded by presentation to a series of screens to remove valuelessfines. The stream from Stage 8 was first presented to pressure-fed 120°sieve bends (200 microns) which retained the fibre product. The waterstream which passed through these screens was presented to 45° weir-fedsieve bends (100 microns) to scavenge residual fibre before recycle. Thefine gangue in the water stream passing this screen was removed from thesystem by passing through a lamellar thickener (Stage 6). The openedfibre product was retreated by passing through small hydrocyclones andsieve bends essentially as done in Stages 8 and 9. This yielded theproduct J (Table 1).

Stage 10. Fibre opening

The poorly-opened intermediate fibre product from Stage 8 was subjectedto a fiberizing treatment, with the aid of a chemical fiberizingreagent, Matexil WA-OT, following the process of U.S. Pat. No.4,226,672.

Well opened fibre was separated from residual poorly opened fibre bypassing through 50 mm hydrocyclones, the latter component being recycledfor further treatment.

Stage 11. Fibre cleaning

The opened-fibre fraction from Stage 10 was cleaned by presentation topressure fed, 120° sieve bends (200 micron aperture) to eliminate finegangue. The retained fibre was the product K (Table 1).

The products J and K were mixed by re-slurrying and dewatering to give asingle blended product from this experiment (Table 1).

The results of measurements made on each of the products from each stageare recorded in Table 1. These results include the yield of eachextracted product as a percentage by weight on the feed ore, the modulusof rupture (MRA) for plaques containing 12.5% of fibre, the Bauer-McNettsizing (top and bottom fractions only shown), and a specific surfacearea (SSA) measured by a water permeability test.

                  TABLE 1                                                         ______________________________________                                                        Sizing (%)                                                    Fibre     Yield   MRA     +14   -325  SSA                                     Product   %       kg/cm.sup.2                                                                           mesh  mesh  Cm.sup.2 /g fibre                       ______________________________________                                        Well-opened                                                                             1.4     443     40    9     33 000                                  product (J)                                                                   Poorly-opened                                                                           2.3     284      2    8      9 000                                  intermediate                                                                  Chemically                                                                              1.9     340     76    9     48 000                                  fiberized                                                                     product (K)                                                                   Blended   3.3     407     63    7     35 000                                  product                                                                       ______________________________________                                    

A matched sample of the same feed material was processed in the corelaboratory at the source of the ore. The standard evaluation of the datashowed that the monetary value of fibre extracted by the wet process wasjust over twice the value of the fibre extracted by the dry process.

EXAMPLE 2

A sample of 700 kg of a Canadian chrysotile ore (precrushed to pass 16mesh) was treated using the equipment described in Example 1 and asimilar circuit. The yield of blended fibre product was 6.8% and it gavea modulus of rupture (MRA) for plaques containing 121/2% fibre, of 320kg/cm².

A matched sample of this feed material was processed in the same corelaboratory (dry process) as for Example 1. The yield was 5.3% of a fibreproduct having a lower reinforcing value than the product from the wetprocess.

EXAMPLE 3

A sample of 400 kg of tailings from the conventional dry milling of theCanadian chrysotile ore used for Example 2 was treated using theequipment described in Example 1 and a similar circuit. The yield ofblended fibre product was 8.3% and the fibre had a modulus of rupture(MRA) for plaques containing 121/2% fibre, of 308 kg/cm².

A matched sample of this feed material was processed in the same corelaboratory (dry process) as for Example 1. The yield was 2.4% of a fibreproduct having reinforcing value slightly lower than the product fromthe wet process.

EXAMPLES 4 AND 5

A sample of the poorly-opened intermediate fibre of Example 1 (20 kg ofcontained fibre) was subjected to fiberizing by wet mechanical meansinstead of by the addition of chemical reagent used for Example 1. As inExample 1 (Stages 10 and 11) the circuit was closed through a 50 mmhydrocyclone and the opened fibre was cleaned over a sieve bend (300micron aperture).

Two fiberizing devices were compared; a rotating disc type colloid milland a rod mill. Measurements made on the fibre product, as for Example1, are recorded in Table 2.

                  TABLE 2                                                         ______________________________________                                                      MRA      Sizing %                                               Example   Fiberizing                                                                              (121/2%)   +14  -325                                      No        device    kg/cm.sup.2                                                                              mesh mesh                                      ______________________________________                                        4         Disc mill 376        11    4                                        5         Rod mill  413         5   14                                        ______________________________________                                    

EXAMPLE 6

A mechanical press was used to demonstrate the removal of water from wetfibre by the application of pressure. The press comprised a metal pistonfitting closely inside a metal ring, approximately 4 cm high and 2 cminternal diameter. The ring was clamped to a metal base plate with asheet of 200 mesh wire screen held between the two parts. The specimenof wet fibre was confined in the ring between the piston and the metalbase plate.

A sample of wet fibre (10.4 g)--the product J of Example 1--containing82% by weight of water was confined in the press and the piston wasloaded to subject the fibre to a pressure of 6.4 MPa. Essentially clearwater exuded between the ring and the base plate. The water content ofthe resulting cake of fibre was 30% by weight. On immersion in water thecake began to break up and was then readily dispersed by stirring.

EXAMPLES 7-10

Four samples of a wet fibre, similar to the product J of Example 1, weresubjected to a series of pressures in a mechanical press, as describedin Example 6. The residual water contents of the compressed cakes arerecorded in Table 3.

                  TABLE 3                                                         ______________________________________                                        Example    Applied Pressure                                                                           Water Content                                         No         MPa          % weight                                              ______________________________________                                        7          3            30                                                    8          14           24                                                    9          21           20                                                    10         34.5         16.8                                                  ______________________________________                                    

The compressed cake of Example 10 was immersed in water. It expanded toabout twice its volume and began to break up, after which it could bedispersed by stirring.

EXAMPLE 11

The process was applied essentially as described to samples of ore atthe Woodsreef Mine (Barraba, New South Wales) during the period Januaryto April 1982. Each sample was also treated with the core laboratory dryprocess. Table 4 records the dry weight of ore, weight and yield (%) ofproduct by both the wet and dry process, the mean FSU (fibre strengthunits for an MRA of 275), and the effective wet/dry yield ratio.

                  TABLE 4                                                         ______________________________________                                                                Dry                                                                           Core Lab.   Effec-                                    Period      Wet Process Process     tive                                      (Sam-  Dry ore  Yield   Mean  Yield Mean  Ratio*                              ples)  (tonnes) %       FSU   %     FSU   Wet/Dry                             ______________________________________                                        1 (1-9)                                                                              19.1     5.26    62    2.72  68    1.74                                2 (10-20)                                                                            64.5     3.13    93    2.63  87    1.27                                3 (21-25)                                                                            52.9     3.64    86    2.67  80    1.47                                4 (26-30)                                                                            70.2     3.10    83    2.38  74    1.46                                5 (31-35)                                                                            22.7     3.40    92    2.81  75    1.50                                6 (36-38)                                                                            27.1     3.28    102   2.64  74    1.71                                ______________________________________                                         ##STR1##                                                                 

Individual yields on samples for the wet process ranged from 4.2-7.9%w/w, and for the dry process, 2.5-3.4% w/w. The yields from the wetprocess are for dried product fibre.

EXAMPLE 12

A similar comparison to that of Example 11 was made for the period Julyto September 1982, except that in the data recorded in Table 5 theindividual samples have been grouped by blasts (and hence particular orelocations) as well as chronological sequence.

                  TABLE 5                                                         ______________________________________                                                            Dry                                                                           Core Lab.   Effec-                                                  Wet Process                                                                             Process     tive                                                       Ton-   Yield Mean  Yield Mean  Ratio                             Blast                                                                              Data    nes    %     FSU   %     FSU   Wet/Dry                           ______________________________________                                        131  Jul     12.6   3.35  118   2.89  80    1.71                                    8-12                                                                     84  Aug     25.0   4.46  80    2.12  90    1.87                                   3-9                                                                       84  Sept    26.2   4.56  81    2.28  85    1.91                                   7-9                                                                      151  Sept    19.6   6.78  78    2.73  88    2.20                                   10-16                                                                    131  Sept    31.4   5.01  77    2.01  93    2.06                                   16-22                                                                    ______________________________________                                    

The average effective wet/dry ratio was 1.90.

EXAMPLE 13

Table 6 records a comparison of the wet process of our invention withboth the dry core laboratory process and the commercial dry mill processfor several batches of ore from the Woodsreef Mine.

                  TABLE 6                                                         ______________________________________                                        Equivalent yield for an FSU of 85                                             Blast    Wet Process    Core Lab Dry Mill                                     ______________________________________                                         94      3.07           2.28     2.63                                         113      4.00           2.23     2.40                                         129/130  3.50           2.38     3.01                                         149/151  6.19           2.81     3.77                                         156      4.38           2.41     3.22                                         ______________________________________                                    

EXAMPLE 14

Samples of waste tailings from the Woodsreef dry mill were subjected tothe wet process of our invention. The yields varied considerably as isshown in Table 7, but in general up to 2% w/w yield (equivalent FSU 85grade) can be obtained from tailings that would otherwise be discarded.

                  TABLE 7                                                         ______________________________________                                                                           Calculated                                                                    Yield equiv                                                                   for                                        Origin     Tonnes  % Yield    FSU  FSU = 85                                   ______________________________________                                        MT         12.4    1.51       72   0.98                                       (May 20-21)                                                                   WMT        14.4    0.85       105  1.05                                       (May 26-27)                                                                   OT 4        9.4    2.14       77   2.10                                       (Jul 6-8)                                                                     OT 6       17.9    2.26       71   2.03                                       (Oct 22-25)                                                                   OT 3       38.3    4.74       65   2.97                                       (Aug 10-25)                                                                   OT 7       47.6    3.56       73   2.01                                       (Aug 26-                                                                      Sept 3)                                                                       OT 8       13.9    2.64       91   1.95                                       (Sept 23-27)                                                                  ______________________________________                                    

EXAMPLE 15

Samples of wet asbestos fibres from the process of our invention weredewatered in a RVP-36 disc drier. At 0.6 rpm and at the maximumoperating cylinder pressure of 2500 psi a 58% w/w solids output wasobtained from a 39.5% w/w solids input. The effluent contained 0.0075%w/w solids. The throughput was 18.5 MT/D. With a speed of 1.5 rpm theoutput fell to 53.4% w/w solids but the rate increased to 33 MT/D.

We claim:
 1. A wet process for the recovery of asbestos fibre fromasbestos-bearing material which process comprises preparing a fibreconcentrate by comminuting said asbestos-bearing material in thepresence of water thereby forming a slurry, separating the fibre bymeans of at least one mechanical classifier and at least one spiralseparator, and concentrating the fibre using at least one hydrocycloneof at least 150 mm diameter; fractionating said fibre concentrate into awell-opened fibre fraction and a poorly opened fibre fraction by meansof at least one hydrocyclone of diameter less than 150 mm; andcollecting the well-opened fibre fraction on a screen.
 2. A wet processfor the recovery of asbestos fibre from asbestos-bearing material whichprocess comprises preparing a fibre concentrate by comminuting saidasbestos-bearing material in the presence of water thereby forming aslurry, separating the fibre by means of at least one hydrocyclone of atleast 150 mm diameter and at least one spiral separator, andconcentrating the fibre using at least one hydrocyclone of at least 150mm diameter; fractionating said fibre concentrate into a well-openedfibre fraction and a poorly opened fibre fraction by means of at leastone hydrocyclone of diameter less than 150 mm; and collecting thewell-opened fibre fraction on a screen.
 3. A process as in claim 1 or 2wherein the asbestos-bearing material comprises chrysoltile.
 4. Aprocess as in claim 3 wherein the said asbestos-bearing material isselected from the group consisting of crude ore, fibre concentrates fromdry and wet processes, mine wastes and tailings, and low grade shortfibre.
 5. A wet process for the recovery of asbestos fibre fromasbestos-bearing material comprising comminuting said asbestos-bearingmaterial; separating said comminuted material into a first crude fibreconcentrate and tailings by means of at least one mechanical classifierand at least one spiral separator; comminuting said tailings to liberatefurther fibre; separating said comminuted tailing into a second crudefibre concentrate and rock tailings using at least one spiral separator;combining said first crude fibre concentrate and said second crude fibreconcentrate and further concentrating the combined concentrates to givea fibre concentrate and fibre concentrate tailings by means of at leastone hydrocyclone of at least 150 mm diameter; fractionating the fibreconcentrate to give a well-opened fibre fraction and a poorly-openedfibre fraction using at least one hydrocyclone of diameter less than 150mm; collecting the well-opened fibre fraction on a screen; fiberizingthe poorly-opened fibre fraction to give a fiberized fraction;concentrating said fiberized fraction to give a cleaned fiberizedfraction and residual poorly-opened fibre tailings by means of at leastone hydrocyclone of diameter less than 150 mm diameter; and collectingthe cleaned fiberized fraction on a screen.
 6. A wet process for therecovery of asbestos-fibre from asbestos-bearing material comprisingcomminuting said asbestos-bearing material; separating said comminutedmaterial into a first crude fibre concentrate and tailings by means ofat least one hydrocyclone of at least 150 mm diameter and at least onespiral separator; comminuting said tailings to liberate further fibre;separating said comminuted tailing into a second crude fibre concentrateand rock tailings using at least one spiral separator; combining saidfirst crude fibre concentrate and said second crude fibre concentrateand further concentrating the combined concentrates to give a fibreconcentrate and fibre concentrate tailings by means of at least onehydrocyclone of at least 150 mm diameter; fractionating the fibreconcentrate to give a well-opened fibre fraction and a poorly-openedfibre fraction using at least one hydrocyclone of diameter less than 150mm; collecting the well-opened fibre fraction on a screen; fiberizingthe poorly-opened fibre fraction to give a fiberized fraction,concentrating said fiberized fraction to give a cleaned fiberizedfraction and residual poorly-opened fibre tailings by means of at leastone hydrocyclone of diameter less than 150 mm diameter; and collectingthe cleaned fiberized fraction on a screen.
 7. A process as in claim 5or 2 wherein the poorly-opened fibre fraction is fiberized in thepresence of a chemical fiberizing agent.
 8. A process as in claim 5 or 2wherein the well-opened fraction comprises fibrous material separatedfrom the asbestos-bearing material by comminution with minimal breakingof the fibrils, and the poorly-opened fraction comprises fibrousmaterial that undergoes substantial fibril-breakage on comminution.
 9. Aprocess as in claim 8 wherein the screen used to recover liberated fibrehas a mesh size of not less than 100 microns.
 10. A process as in claim5 or 2 wherein the means of comminution of the poorly opened fractioncomprises a ball mill.
 11. A process as in claim 1 or 6 wherein thefibre concentrate tailings are recycled and subjected to furthercomminution.
 12. A process as in claim 11 wherein the recycled tailingsadditionally comprise the residual poorly-opened fibre tailings.
 13. Aprocess as in claim 1 or 6 wherein the screen comprises a sieve bend.14. A process as in claim 13 wherein the screen used to recover thefiber concentrate has a mesh size of not less than 300 microns.
 15. Aprocess as in claim 5 or 6 wherein the means of comminution of thepoorly opened fraction comprises a rod mill.
 16. A process as in claim 5or 6 wherein the means of comminution of the poorly opened fractioncomprises a rotating-disc type colloid mill.